Bioinspired Heme and NonHeme Metal Complexes as Electrocatalysts for Nitrate and Nitrite Reduction

Excess nitrate (NO 3 − ) and nitrite (NO 2 − ) from agricultural and industrial sources are major water pollutants and precursors of greenhouse gases. The electrocatalytic reduction of NO 3 − /NO 2 − is regarded as a sustainable solution for the remediation of contaminated water and a method for producing ammonia (NH 3 ) under ambient conditions. In this review, molecular electrocatalyst macrocycles derived from heme, such as porphyrins and phthalocyanines, and nonheme complexes for NO 3 − /NO 2 − reduction are presented. Based on enzymes such as molybdenum‐dependent nitrate reductases and copper‐ or heme‐containing nitrite reductases, representative synthetic enzymes are discussed in terms of their mechanisms, catalytic parameters, ligand effects, product selectivity, and stability. The conversion of NO 2 − to NH 3 is the most prominent application of heme‐type Fe and Co macrocycles Nonheme systems, such as Cu polypyridines, Ni diamine, or Fe/Co N‐donor complexes, can also produce NO, N 2 O, or NH 3 by varying the ligand environment or pH. We identified some advances, such as high turnover frequencies, elucidation of the competitive proton‐coupled electron transfer (PCET) and atom transfer pathways, and new strategies for increased durability. Finally, the principles of second‐sphere proton relay and electronic tuning are discussed, and the opportunities for embedding macrocycles within conductive frameworks and emulating multisite enzymatic architectures to create efficient and robust electrocatalysts for nitrogen oxide (NO x ) remediation and green NH 3 production are outlined.